The invention pertains to visual displays and more particularly to electrical twisting-ball displays, such as gyricon displays and the like.
Gyricon displays, also known by other names such as electrical twisting-ball displays or rotary ball displays, were first developed over twenty years ago. See U.S. Pat. No. 4,126,854 and No. 4,143,103, incorporated by reference hereinabove.
An exemplary gyricon display 10 is shown in side view in FIG. 1 (PRIOR ART). Bichromal balls 1 are disposed in an elastomer substrate 2 that is swelled by a dielectric fluid creating cavities 3 in which the balls 1 are free to rotate. The balls 1 are electrically dipolar in the presence of the fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes 4a, 4b. The electrode 4a closest to upper surface 5 is preferably transparent. An observer at I sees an image formed by the black and white pattern of the balls 1 as rotated to expose their black or white faces (hemispheres) to the upper surface 5 of substrate 2.
Gyricon displays have numerous advantages over conventional electrically addressable visual displays, such as LCD and CRT displays. In particular, they are suitable for viewing in ambient light, retain an image indefinitely in the absence of an applied electric field, and can be made lightweight, flexible, foldable, and with many other familiar and useful characteristics of ordinary writing paper. Thus, at least in principle, they are suitable both for display applications and for so-called electric paper or interactive paper applications, in which they serve as an electrically addressable, reuseable (and thus environmentally friendly) substitute for ordinary paper. For further advantages of the gyricon, see U.S. Pat. No. 5,389,945, incorporated by reference hereinabove.
Although gyricon displays promise to offer many of the advantages of ordinary paper together with the advantages of electrically addressable displays, the gyricon displays of the prior art have not lived up to their promise. Simply put, these displays do not look as good as paper. In particular, they do not have the high reflectance of paper (typically, 85 percent diffuse reflectance for white paper) and, consequently, do not have the high brightness and contrast characteristics of paper.
Conventional wisdom holds that the best way to improve the reflectance of a gyricon display is to make the display from a thick arrangement of bichromal balls. It is thought that the thicker the arrangement of balls, the better the reflectance and the brighter the appearance of the display. The intuitive analogy here is to ordinary paint: Other things being equal, a thicker coat of white paint reflects more incident light than a thinner coat of paint, and therefore appears brighter and whiter than the thinner coat. By analogy, it is expected that a thick arrangement of bichromal balls will tend to reflect more incident light than a thinner arrangement. In particular, the white faces of bichromal balls located at some distance below the viewing surface of the display are expected to reflect any light that is not reflected by balls located nearest the surface.
Conventional wisdom also suggests that to achieve high resolution in a gyricon display, the cavities in which the balls rotate should be packed as closely together as possible. However, it is conventionally supposed that the size of the balls within the cavities is of no consequence insofar as display reflectance is concerned. That is because in a display having a thick arrangement of bichromal balls, the balls located farther from the viewing surface of the gyricon display will "fill in the gaps" between bichromal balls located nearer the viewing surface. In other words, so long as the two-dimensional projection of the balls at all distances from the viewing surface onto the viewing surface substantially covers the viewing surface, a high-quality display will be obtained.
The series of views of FIG. 2 (PRIOR ART) illustrates several different thick arrangements of bichromal balls found in various gyricon displays of the prior art. View (a) shows first arrangement 210, which is made up of bichromal balls 211 in spherical cavities 212 arrayed so as to form multiple layers 217, 218, 219. View (b) shows second arrangement 220, which is made up of bichromal balls 221 in spherical cavities 222 arrayed in wavy layers 227, 228, 229; as can be seen, the division of second arrangement 220 into layers 227, 228, 229 is somewhat arbitrary. View (c) shows third arrangement 230, which is made up of bichromal balls 231 in spherical cavities 232 that are not layered at all, but instead are distributed randomly throughout the thickness of arrangement 230.
A thick display has certain drawbacks. Notably, a thinner display should require a lower drive voltage. Nevertheless, in keeping with the conventional wisdom, virtually all known gyricon displays are made with thick arrangements of bichromal balls (e.g., sheets of bichromal balls wherein the sheets are several ball diameters thick), because this is thought to be necessary in order to produce displays of adequate brightness. The following references are noteworthy in this regard:
application Ser. No. 08/368,133, commonly assigned with the present invention, includes a passing reference to an apparently hypothetical gyricon display containing a monolayer of bichromal balls: "Typically, for a sheet of Electric Paper containing a monolayer of bichromal balls with an average diameter of 80 microns, 50 volts will be applied to . . . orient the balls in a common direction . . . " (Specification, p. 6). However, this example appears to be posed to illustrate the calculation of appropriate drive voltages in the disclosed gyricon display, so as to show a theoretical minimum voltage needed to drive such a device. The remainder of the disclosure contemplates the usual thick ball arrangements (see, e.g., FIGS. 8 and 9 of the Ser. No. 08/368,133 disclosure). PA1 A paper by Lee entitled "A Magnetic-Particles Display", IEEE Transactions on Electron Devices, Vol. ED-22, No. 9, September 1975, pp. 758-765, concerns a magnetically activated twisting-ball display. At page 762, under the heading "Resolution, Contrast, and Gray Scale," Lee discusses the contrast provided by single and multiple layers of particles. He proposes that a random packing arrangement of bichromal balls is optimal, and predicts improved reflectance with a double layer (15 to 60 percent reflectance predicted) as opposed to a single layer (0 to 45 percent reflectance predicted).
If the conventional wisdom were correct, a display such as that disclosed by Ishikawa, Saito, Mori, and Tamura of Sony Corporation, in a U.S. Patent issued to these persons (U.S. Pat. No. 4,438,160, hereinafter the '160 patent) and assigned to Sony and further in a paper entitled "A Newly Developed Electrical Twisting Ball Display," Prioceedings of the SID, Vol 23/4, 1982, pp. 249-253, ought to produce good contrast and brightness. Ishikawa and his colleagues disclose a twisting-ball display having multiple layers of bichromal balls disposed in cavities whose walls touch each other (see '160 patent at col. 6, lines 8-15). They argue that by arranging the balls with a high packing density, high resolution displays can be achieved (see '160 patent at col. 7, lines 10-12).
Note that there is no discussion in the '160 patent of the relative sizes of the balls and the cavities. FIG. 6 and FIGS. 12-13 of the '160 patent are not to the contrary. In particular, the specification describing FIG. 6 and FIGS. 12-13 of the '160 patent makes no mention whatsoever of the relative diameters of balls and cavities. Moreover, the technique disclosed for forming the cavities (coating the balls with wax and later dissolving away the wax) suggests that the relative dimensions illustrated in FIG. 6 and in FIGS. 12-13 of the '160 patent are misleading, and that in practice, the cavities will be considerably larger than the balls. The '160 patent does not specify the particular method for deposition of wax on the balls (see '160 patent at col. 4, lines 60-65), but deposition of wax ("resin") by the technique disclosed at page 251 of the Saito et al. paper would produce a wax coating ranging from 5 to 15 microns in thickness, so that the cavity diameter would range from 1.2 to 1.6 times the specified 50-micron ball diameter (see '160 patent at col. 4, line 14; Saito et al. paper at p. 251).
The suggestion that the cavities in the device contemplated by the '160 patent would be substantially larger than the balls they contain is borne out by a photomicrograph of an actual device built according to the principles set forth in the '160 patent. In this photo, which is shown in FIG. 2 on page 250 of the Saito et al. paper, a thick arrangement of bichromal balls with sizeable gaps between the balls located closest to the surface can be seen. Of course, the conventional wisdom teaches that these gaps ought not to matter from the standpoint of display reflectance. The balls located farther from the viewing surface of the gyricon display should effectively "fill in the gaps" between bichromal balls located nearer the viewing surface, so that the quality of the overall display is not impaired. Put differently, it should not matter whether incident light is reflected from balls closest to the viewing surface or from balls situated a greater distance away from the viewing surface, so long as the incident light is reflected somehow.
As it turns out, the display proposed by Ishikawa and his colleagues does not have especially good reflectance properties, at least when compared with ordinary paper. Indeed, to date, known gyricon displays have offered at most about 15 to 20 percent reflectance (as measured when the white faces of all bichromal balls are turned towards the observer).
So a puzzle remains: Why do gyricon displays, even those of high resolution, lack the reflectance, brightness, and contrast qualities predicted by the conventional wisdom? How can a gyricon display be made that has superior reflectance, contrast, and brightness? Without these qualities, the promise of the gyricon display--to make electric paper a working reality instead of a laboratory curiosity--will remain unfulfilled.